In many circuit arrangements, a logic signal must be transmitted between two circuits that must otherwise be electrically isolated from one another. For example, the transmitting circuit could utilize high internal voltages that would present a hazard to the receiving circuit or individuals in contact with that circuit. In the more general case, the isolating circuit must provide both voltage and noise isolation across an insulating barrier.
Such isolation circuits are often referred to as “galvanic isolators”. One class of galvanic isolators is based on transforming the logic signal to a light signal that is then transmitted to an optical receiver in the receiving circuit that converts the optical signal back to an electrical signal. The transmitting and receiving circuits are typically on separate substrates and connected to separate power supplies. Unfortunately, when there is high transient noise at both the low voltage and high voltage grounds, unwanted noise is unavoidably coupled from the input to the output.
High voltage common mode transient immunity (CMR) is a critical parameter of an isolator, as common mode noise problems exist in many electrical circuits. Prior art galvanic isolators rely on minimizing the parasitic capacitance between the anode of the LED in the transmitter and the ground of the photodetector on the receiver side of the isolator to minimize the common mode transient rejection of the isolator. Unfortunately, this approach still leads to unacceptable common mode transient rejection in some applications.